KARBALA, Iraq -- Posted at the door of Shiite cleric Moqtada al-Sadr's office recently, a flier denounced the arrests of his followers. Up and down the barricaded street, soldiers and policemen loyal to his Shiite rivals stood sentry, some in tan armored personnel carriers, questioning anyone they suspected of links to the populist cleric.

Inside the shuttered office, five guards spoke frankly of their sense of vulnerability and weakness. Once in control of the streets of this southern city of holy sites, the Sadrists said they have been chased underground, their rivals at their heels.

The arrests of Sadr's loyalists are part of a broader power struggle between the two most powerful Shiite factions seeking to lead Iraq: the Sadrists, who are pushing for U.S. troops to withdraw, and the Islamic Supreme Council of Iraq, the Bush administration's main Shiite ally. Given the nation's majority-Shiite population, this intensifying confrontation could play a major role in deciding Iraq's future.

This year's U.S. military offensive and dramatic shifts in tactics by both Sunni and Shiite groups are redrawing the balance of power across Iraq. With less violence between Sunnis and Shiites, festering struggles within each community may come to define the nature of the conflict. In the Shiite-dominated south, Sadr's main Shiite rivals are taking advantage of the surge in U.S. troops, as well as Sadr's imposition of a freeze on operations by his Mahdi Army militia, to make political gains.

"They are all gathering against us," said Ayad Abu Ali, a wiry, broad-shouldered militia guard who had sent his family into hiding and now hardly leaves the office.

U.S. forces have arrested hundreds of Mahdi Army militia members in Baghdad, creating voids in the leadership. This has emboldened Iraq's mostly Shiite security forces, loyal to the Supreme Council and other political parties, to reach for power in the south. In cities such as Karbala, Diwaniyah and, most recently, Hilla, scores of Sadr's followers are routinely being detained.

"If this American pressure did not exist on the Mahdi Army in Baghdad, of course Iraqi security forces would not be able to make these arrests in the southern provinces," said Abdul Hadi al-Mohammadawi, a cleric who heads Sadr's operation in Karbala. "And if the freeze did not exist, this would not be happening."

Struggle in the South

In the southern holy city of Najaf, pilgrims sat against a wall of the Imam Ali shrine, one of Shiite Islam's most sacred sites. On the ornate facade is a white patch in the shape of the face of Ayatollah Mohammed Bakir Hakim. Assassinated by a car bomb in August 2003, Hakim was the leader of the Supreme Council. His younger brother, Abdul Aziz al-Hakim, one of Iraq's most powerful politicians, now heads the party.

Six months ago, Supreme Council loyalists sought to create a portrait of the elder Hakim in brickwork on the shrine's facade. But Sadr's followers took to the streets and stopped them from finishing the project.

"Each side is determined to be in control of the south," said Mohammed Jassim, a prominent tribal leader in Diwaniyah province.

The competition has its origins in the days when the fathers of Hakim and Sadr, both preeminent ayatollahs, fought to lead Iraq's Shiites. Under Saddam Hussein, Abdul Aziz al-Hakim spent years in exile in Iran. Sadr remained in Iraq, bolstering his street credentials. After the U.S. invasion in 2003, Hakim embraced the Americans, while Sadr went to war against them, launching two major uprisings in Najaf in 2004.

Today, their struggle is multidimensional, playing out along lines of personality, class and ideology. The contest is a street fight over turf, a tug of war over oil revenues and a battle for control of the shrines. Sadr's militia has targeted Hakim's party offices and fought his movement's armed wing, the Badr Organization. Both militias are widely believed to have operated death squads targeting each other and Sunnis.